Generally, as an image reading device of such a type, there is known an image reading device as disclosed in Patent Document 1 and Patent Document 2, configured to irradiate a document fed onto or placed on a reading platen with linear light emitted from a light guide and to receive reflected light therefrom by a photoelectric conversion sensor, thereby reading an image of the document.
An illumination device mounted in the image reading device disclosed in Patent Document 1 has a light source unit and a photoelectric conversion sensor on a scanning carriage that moves along a reading platen and receives reflected light of light emitted from the light source unit by the photoelectric conversion sensor. The photoelectric sensor, which is configured as a line sensor, reads an image on the platen in a line-sequential manner while moving in a scanning carriage moving direction (sub-scan direction) perpendicular to a reading direction (main-scan direction) of the photoelectric conversion sensor.
The light source used as a light source of the illumination device includes a white LED light source serving as a light emission source and a light guide that diffuses light from the light source uniformly in the main scan direction to serve as a rod-like luminous body and is required, in this case, to emit light uniformly in the main scan direction (longitudinal direction of the line sensor).
In order to meet the above requirement, the light source unit is configured as follows: the light guide is formed into a rod-like shape having a diffusion reflecting surface and a light exiting surface which are disposed opposite to each other in the can direction; a light source is provided at one end portion of an outer cover in which the light guide is housed; one end surface of the light guide housed in the outer cover is made to face the light source with a predetermined gap provided therebetween; and a reflecting surface serving as a pseudo light source (although a detailed description thereof is omitted) is formed at the other end surface of the light guide. The configuration described above allows light emitted from the light guide to uniformly enter the reading platen.
Further, an inner side surface of the outer cover at the one end portion thereof having the light source is sloped to allow the outer cover itself to serve as a reflector for guiding the light emitted from the light source to the light guide, thereby allowing the light from the light source to efficiently enter the light guide.
Further, the image reading device disclosed in Patent Document 1 includes so-called a contact type optical system in which a SELFOC lens is provided for each sensor element in the line sensor. On the other hand, in an image reading device of so-called an optical reduction system that uses a condenser lens like an image reading device disclosed in Patent Document 2, intensity of light at both end portions of a reading area in the main scan direction is reduced according to the cosine fourth power law. In order to cope with this, in an image reading device of such a type, the gap between the light guide and light source and a position of the light source facing the light guide are adjusted so as to increase light intensity distribution characteristics at an irradiation surface (reading surface) by an amount corresponding to the attenuation caused due to the cosine fourth power law.
Thus, the light intensity distribution characteristics of the illumination device using the light guide need to be managed depending on usage thereof. Particularly, as described above, when the illumination device is used as the illumination device for the image reading device, the light intensity distribution characteristics thereof need to be managed such that the light emitted from the light guide and illuminating the reading platen becomes uniform linear light in the case of the illumination device of Patent Document 1 and such that the light emitted from the light guide and illuminating the reading platen becomes linear light having an intensity greater at the both end portions of the reading area in the main scan direction than at a center thereof in the case of the illumination device of Patent Document 2. In order to meet a condition that maintains a state the liner light based on the preciously set intensity distribution, it is essential to take into consideration, at designing, maintenance of the predetermined gap between the end surface of the light guide housed in the outer cover and the light source serving as a light emission source, position of the light source facing the end surface of the light guide, a shape of the light guide, light directivity of the light source, and the like.